Multifocal ophthalmic lenses are now well known. They are normally used for correcting presbyopia, while enabling the wearer of spectacles to observe objects over a wide range of distances, without requiring to remove the spectacles to see distant objects. In conventional multifocal opthalmic lenses the term "power addition" or, in short "addition", is used to indicate the increase in optical power between the first and second above-mentioned points on the main meridian curve. Usually, the manufacturers of multifocal opthalmic lenses provide a family of half-finished lenses, i.e. for lenses of the same family, the lens surface including the three above-mentioned vision zones is machined initially on its own, with the other surface of a lens in the family being subsequently machined to a sphericl or toroidal shape having curvature appropriate to each wearer of spectacles, depending on the prescriptions of an opthalmologist. Within a family of lenses, power addition varies gradually from one lens to another in the family between a minimum addition value and a maximum addition value. Usually, the minimum and maximum addition values are respectively 0.5 diopters and 3.5 diopters, and addition varies in 0.25 diopter steps from one lens to the next within a family. In this case, a family of lenses comprises 13 lenses.
The multifocal opthalmic lenses which are commercially available at present come in two main families. In the first family of lenses, progression length, i.e. the distance between the first and second above-mentioned points on the main meridian curve is constant, and the optical power progression gradient varies from one lens to another in said first family. More precisely, the higher the power addition, the faster the increase in optical power along the main meridian curve between the first and second points thereof. An example of multifocal opthalmic lenses in the first family is represented by "VARILUX 2" lenses manufactured by the Applicant (French patent FR No. 2 058 499 and its two certificates of addition FR No. 2 079 663 and FR 2 193 989, equivalent to U.S. Pats. Nos. 3,687,528 and 3,910,691.
In the second family of lenses, the gradient of optical power progression along the main meridian curve is constant and identical for all of the lenses in said second family, regardless of their power addition. In this case, the progression length increases linearly with the value of the power addition. An example of lenses in the second family is described, for example, in Japanese patent No. JP 54-85743.
It is well known that regardless of the family to which they belong, multifocal opthalmic lenses inevitably suffer from optical aberrations (astigmatism, distortion, field curvature, etc.) which reduce visual comfort both in static vision and in dynamic vision. In addition, as the presbyopia of a long-sighted person increases, requiring the use of lenses havig increasing power addition, the changeover to lenses having greater power addition usually requires an effort of physiological adaptation on the part of the spectacle wearer. The adaptation time may be from one to several days depending on the person.
In the past, manufacturers of multifocal opthalmic lenses have directed their efforts mostly to improving visual comfort. To this end, several means have already been proposed and used either separately or in combination, namely:
(a) A suitable choice of progression law for the optical power along the main meridian curve.
(b) Optical modulation, i.e. distributing optical powers over the side portions of the surface of the lens, for example by an appropriate selection of main curvatures of the surface along its horizontal sections (French patent No. FR 2 058 499, equivalent to U.S. Pat. No. 3,687,528.
(c) Reducing surface distortion so as to satisfy the orthoscopic condition (horizontal lines and vertical lines in the field of view remain horizontal and vertical). This may be obtained, for example, by providing the surface of the lens with one or more horizontal umbilical lines and one or more vertical umbilical lines (with the main radii of curvature of the surface at each point along such lines having the same value) and/or one or more horizontal lines and one or more vertical lines along which the prismatic effect is constant in value (at each point along a line of constant prismatic effect the plane tangential to the surface makes a constant angle with a plane which is horizontal or vertical). See French certificate of addition FR No. 2 079 663, equivalent to U.S. Pat. No. 3,687,528.
(d) Improving suitability for binocular vision by inclining the main meridian curve from top to bottom of the lens surface away from the temporal region and towards the nasal region, and making the surface in such a manner that any two points on the surface which are equidistant from the main meridian curve in the horizontal direction have the same optical characteristics (see the above-mentioned French patents).
(e) Improving visual comfort for dynamic vision (see French certificate of addition FR No. 2 193 989, equivalent to U.S. Pat. No. 3,910,691.
However, up till now, it does not appear that the problem of reducing the effect of physiological adaption and the adaptation time when changing from one pair of lenses having a first value of power addition to another pair of lenses having a second, higher value of addition has been solved in a satisfactory manner, nor even that any attemmpt has been made to solve this problem.
Starting from a surface structure providing optical modulation and providing satisfactory overall visual comfort, the object of the present invention is to provide a multifocal opthalmic lens belonging to a family which is different from the above-mentioned first and second families of lenses, providing improved visual comfort for a person suffering presbyopia, regardless of the value of power addition of the lens, with the family of lenses of the present invention requiring a smaller effort of physiological adaptation and a shorter adaptation time when a person whose presbyopia is increasing changes lenses, going from one pair of lenses in accordance with the invention having a first value of power addition to another pair of lenses in accordance with the invention having a second, higher value of power addition.
After a long period of investigation and numerous tests performed on a sample on about 200 people, the Applicant has discovered that this object can be obtained.